debian: enable xfs_healer on the root filesystem by default

debian: enable xfs_healer on the root filesystem by default

Now that we're finished building autonomous repair, enable the healer
service on the root filesystem by default.  The root filesystem is
mounted by the initrd prior to starting systemd, which is why the
xfs_healer_start service cannot autostart the service for the root
filesystem.

dh_installsystemd won't activate a template service (aka one with an
at-sign in the name) even if it provides a DefaultInstance directive to
make that possible.  Hence we enable this explicitly via the postinst
script.

Note that Debian enables services by default upon package installation,
so this is consistent with their policies.  Their kernel doesn't enable
online fsck, so healer won't do much more than monitor for corruptions
and log them.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>